Pulsed lasers are used widely in applications such as microvia drilling, materials marking, stereo lithography, and biosciences. In many of these applications, both precision and flexibility of operation are important. Precision requires that the laser have consistent and essentially identical beam propagation properties and the same laser pulse energy from pulse to pulse. Flexibility includes being able to provide complex pulse trains with variable time periods therebetween. The time period between pulses can range from a tens of microseconds (μs) to a second or greater.
An optimum combination of precision and flexibility includes the capability to deliver laser pulses in a preferred timing sequence with constant or variable (yet consistent) laser pulse energy and nearly identical beam propagation properties. Consistent beam propagation properties are required so that the laser pulses can be consistently focused into a desired spot size at a predetermined location.
Continuously pumped, repetitively Q-switched, solid-state lasers (diode-laser-pumped lasers, in particular) are preferred pulsed laser sources for above-mentioned applications. This is because such lasers have high efficiency, long lifetime, and high reliability. In such a solid-state laser, thermal effects in the laser gain-medium due to optical pumping require particular attention in designing the laser. Significant thermal effects include thermal-lensing and thermal-birefringence, resulting, respectively, from thermal gradient and thermal stress. It is possible to configure a resonator for such a solid-state laser to compensate for or accommodate a predetermined range of thermal effects and minimize variation of laser properties due to changes in these thermal effects within that predetermined range. This can be done in a fixed laser resonator by suitable selection of resonator parameters. It is also possible to have a resonator that is dynamically reconfigurable to accommodate for measured or predicted thermal-lensing changes. Even in such a compensated resonator, if there is a significant difference in thermal-lensing conditions in a gain-medium during and between delivery of pulse trains, the pulses in a train can vary in amplitude and beam propagation properties in a manner that depends on the timing sequence of the laser pulses. Compensation can be insufficient (in the case of a fixed resonator) or too slow (in the case of a dynamically compensated resonator).
In prior-art continuously pumped, repetitively Q-switched solid-state lasers, the Q-switch remains closed between delivery of laser pulses. The term closed, here, means that the Q-switch causes high loss in the resonator so that lasing is prohibited. In such a prior art laser, thermal load on the gain-medium varies with pulse repetition frequency (PRF). Consequently the above-discussed thermal effects, among which thermal lensing usually plays a prominent role in affecting the laser spatial mode and stability, will also vary.
Thermal lensing occurs due to a spatial variation in refractive index of the solid-state gain-medium resulting from a thermal gradient in the gain-medium. This thermal gradient results from heating of the gain-medium by a portion of pump power absorbed therein that is not extracted as laser radiation and other factors. In an end-pumped gain-medium, thermal lensing is proportional to the difference between the total pump power and the power extracted from the gain-medium as laser radiation. The power extracted from the gain-medium depends in turn on the PRF.
By way of example, in a prior-art continuously pumped, repetitively Q-switched solid-state laser, using Nd:YVO4 as the gain-medium, end-pumped with 27 W of diode power, the average laser power extracted from the gain-medium may be about 12.6 W at 50 KHz and 4.6 W at 5 KHz, respectively. Accordingly, net heating power and therefore thermal-lensing will be proportional to 14.4 W and 22.4 W at 50 KHz and 5 KHz PRF. It can be seen that significant thermal lensing variation can occur and can result in significant change in laser spatial mode if the laser is switched between the two different PRFs. This can result in a problem in providing precise machining or marking for example. The problem is exacerbated if pulses of fundamental radiation from the resonator are frequency converted, for example, to generate second, third, or fourth harmonic wavelengths, in one or more optically-nonlinear crystals before being used in a particular application. This is because such frequency conversion always amplifies any variation in the fundamental beam. Accordingly, there is a need for a method to overcome variations of the thermal effects in the gain-medium when the laser is switched between significantly different PRFs.
In PCT Application WO 01/28050, published Apr. 19, 2001, and assigned to the assignee of the present invention, a method of operating a pulsed laser is described in which the gain-medium is continuously-pumped at a constant level and the laser is operated, between delivery of trains of pulses having sufficient energy to perform a machining operation or other application, to provide “thermal-lensing-control” pulses having insufficient energy to perform the machining operation or application. The rate at which the thermal-lensing-control pulses are delivered is higher than the rate at which the trains of machining pulses are delivered, and is adjusted such that the average laser power extracted from the continuously-pumped gain-medium is about the same as that extracted during delivery of the machining pulses. In one arrangement, the laser is operated in a continuous-wave (CW) mode between delivery of machining pulses with the CW power being about the same as the average power during delivery of the machining pulses. By extracting about the same amount of average laser power from the gain-medium between and during periods in which machining pulses are delivered thermal lensing in the gain-medium can be maintained in the range for which the laser resonator is compensated. While the method is generally effective it is not always possible to eliminate thermal-lensing changes effectively in the transition to delivery of machining pulses from delivery of non-machining pulses (or CW laser beam). This is particularly true when machining pulses are delivered at relatively slow rate for the delivery period, for example less than 10 KHz, and between machining-pulse delivery periods the laser is operated at high PRF or CW. This is because the average power extraction from the gain-medium for the two sequences is significantly different and accordingly the thermal lensing change will result in variation in laser pulse energy or laser beam propagation properties for the pulses delivered. Operation of the laser is further complicated by the fact that, even when it is possible to equate thermal lensing between and during machining-pulse delivery periods, it is necessary to allow a particular delay between the delivery of thermal-lensing-control pulses (or CW laser beam) and delivery of machining pulses. This delay provides that the energy stored in the gain-medium at the instant of delivering the first machining pulse in a train is the same as it will be at the instant of delivering every other pulse in the train. There is a need for a less complicated method of operating a solid-state laser to deliver trains of machining pulses.